Organic light-emitting diodes (“OLEDs”) are self light-emitting devices and are advantageous in that they not only have wide viewing angles and excellent contrasts, but also have fast response times and excellent luminance, driving voltage and response rate and enable demonstration of multicolors.
The driving principle of OLEDs is as follows. When a voltage is applied between a cathode and an anode, holes injected from the bottom electrode are moved to a light emitting layer via a hole transport layer, and electrons injected from the cathode are moved to the light emitting layer via an electron transport layer. The holes and electrons (carriers) are recombined in a light emitting layer to form excitons, and light is emitted as the excited molecules transit from an excited state to a ground state.
OLED performance has been greatly improved recently to demonstrate a high external quantum efficiency value of 29% with very low efficiency roll-off. However, large number of OLEDs reported in literature showed high driving voltages compared to theoretical values. One of the reasons why the OLEDs have high driving voltages is related to an injection barrier of electrons and/or holes between the electrodes and organic layer and/or between organic layers.
For the reduction of driving voltages, there have been attempts to lower or remove the energy barriers and to balance the electrons and holes in the emitting layer without electrical loss. Even with such effort, however, light emitting efficiencies are still low with high driving voltage and high efficiency roll-off at high luminance compared to theoretical values.
Since high efficiency OLEDs reported in literature have not reached satisfactory driving voltage and luminous efficiency levels at the same time as described above, various technologies for solving these problems are desired.